In Reply We appreciate the letters by Drs Marcus, Couraud et al, and Gelbman and Libby in response to our recent study on estimating overdiagnosis in low-dose computed tomographic screening for lung cancer.1 We agree with Dr Marcus that not all cases of overdiagnosis in a cancer screening program are necessarily caused by indolent tumors and should have made this clearer in our article. Given the natural history of most clinically apparent lung cancers, however, we estimate that the majority of overdiagnosis cases in the National Lung Cancer Screening Trial (NLST) were indolent tumors but acknowledge that this may not be exactly the case in other cancers or in population-based screening programs, with extended follow-up.